Laminated thermoplastic billets are used in thermoforming and solid-phase forming. These laminated billets are made by bringing molten layers of thermoplastic together under pressure where the molten layers are in the shape of the billet or where the layers are in the form of a sheet from which the laminated billet is subsequently cut.
Extrusion of thermoplastics into sheets and/or films (hereinafter sheet and film are collectively called sheet) is an old art. Simultaneous extrusion of multiple thermoplastics into sheets is also known. Such simultaneous extrusion is the general method by which laminated sheets are made. The molten individual sheets are brought together under pressure, usually between one or more rollers, and the sheets adhered to each other.
The layers in a laminate must adhere to each other. Some sets of thermoplastics are compatible with each other and can, by melting the contacting surfaces, be made to form a satisfactory laminate bond. Other sets of thermoplastics are not compatible and will not form a satisfactory laminate bond. These latter materials require an adhesive or tie layer between them. Such tie layers are generally known. For example, when polyolefins and polar polymers are to be laminated, one may use a graft copolymer of the polyolefin and maleic anhydride as the tie layer to tie the polyolefin to many polar thermoplastics. In essence, the polyolefin is laminated to one side of a sheet of the maleic anhydride/polyolefin graft copolymer and the polar thermoplastic is grafted to the other side of this tie layer. For example, a graft copolymer of polypropylene with about 0.1-20 percent maleic anhydride will serve as the tie layer between polypropylene and copolymers of ethylene and vinyl alcohol, polyvinyl chloride, nylon, polymethylmethacrylate, polyvinylacetate, and many other polar polymers. In general, tie layers are thinner than their adjacent layers.
If you are laminating two polyolefin layers, it is possible to use a graft or block copolymer of the two olefins as the tie layer or tie billet.
For example, a laminate having a layered sequence of polypropylene, a tie layer (a maleated polypropylene), an ethylene/ vinylalcohol copolymer, a tie layer and polypropylene can be used to form containers having good oxygen and water barrier properties. The polypropylene gives the desired water barrier and the copolymer gives the oxygen barrier. In addition, the laminated billet may be thermoformed or solid-phase formed so as to obtain the required orientation of the polymer and thereby impart the required stiffness to the container.
The prior art method of forming the billets is by hot extrusion of multiple sheets of thermoplastic, lamination of the heated sheets as they exit the extruder while still in the molten state, and thereafter cutting the billets from the cooled laminated sheet. The portion of the sheet that is not used constitutes a laminated web of relatively little value. In most instances, this web is recycled into the laminate sheet and billet by grinding it into small pieces and extruding it as one of the layers or as part of another layer which forms a portion of the laminate.
The multi-thermoplastic regrind layer can be a waste of valuable polymer, and a waste of energy since a layer of blended polymer will not give the properties of the individual layers. Often these blends of thermoplastics are difficult to extrude and/or form.
When a regrind web of the laminate sheet is used as one of the layers in the laminate, it is an ineffective use of the three polymers and may require the use of additional tie layers. Furthermore, in the forming of the laminate and the container, this regrind layer or a layer containing a substantial amount of regrind is difficult to extrude and difficult to form, not leading to the optimum properties for the container. The thermal energy lost as the regrindable web cools also affects the process costs.
As an alternative to the normal oven heating of the sheets, some processes utilize radio frequency or microwave heating processes. It is well known that some polymers can be made to dielectrically heat, or to dielectrically heat more readily, by incorporating dielectrically heatable materials into the polymer. Dielectrical heating is a forty year old art and generally information concerning the techniques used may be found in "Industrial Microwave Heating" by A. C. Metaxas and R. J. Meredith (Peter Peregrinus Ltd. Publisher) and "Plastic Fabrication by Ultraviolet, Infrared, Induction, Dielectric and Microwave Radiation Methods" by Arthur F. Readdy, Jr. (Published by Picatinny Arsenal, Dover, N.J. 07801, Department of Defense April 1972).
Materials like fillers, fibers, additives and plasticizers can be blended into the polymer to enhance its dielectrical heatability. Other polymers and copolymers having polar moieties distributed alone or in the chain (unless symmetrical) will dielectrically heat, e.g. ethylene/vinyl alcohol copolymers, polyvinylidene chloride, vinyl chloride, polyvinylidene fluoride, polyvinyl acetate, ethylene/CO copolymers, nylons, PET, cellulose, and polyurethanes for example.
Other polymers, particularly polyolefins and copolymers of olefins are essentially transparent to radio frequency (0.1.congruent.300 MH.sub.z) and microwave radiation (.about.300-10,000 MH.sub.z), and will not dielectrically heat, e.g. polyethylene, polypropylene, polybutene-1, polypentene, and polystyrene.
Dielectric heating of thermoplastics is essentially based on the interaction of a polar moiety within the thermoplastic with an applied alternating field. This moiety may be a pendent polar group including but not limited to --OH, F, Cl, and ##STR1## in polyvinyl alcohol, polyvinyl fluoride, polyvinyl chloride, polyvinylidene fluoride, nitrile, or a polar moiety incorporated into the polymer chain such as in polyesters ##STR2## and nylons. On the other hand, the responding polar moeity may be an additive to the thermoplastic, including but not limited to solids like talc, glass, carbon black, CaCO.sub.3, or impact modifiers such as ABS, chlorinated PE, nitrile rubber, acrylonitrile, etc., or soluble or compatible chemicals like plasticizers (epoxy resin, dioctylphalate, glycerol, diotylsebacate), and di-2-ethxylphalate.
Simplistically, dielectric heating can be pictured as frictional heating created by the movement of these polar moieties through a viscous medium, i.e. the surrounding polymer mass. Most polar moieties will respond to a frequency range within the radio frequency (RF) and/or microwave (MW) bands. The response is usually over a range of frequency and the maximum response will vary with temperature and with the viscosity of the polymer. In general, many thermoplastics respond better at higher temperatures for a given frequency. The range of heat absorption (P) by a thermoplastic is described as follows: EQU P=2.pi.fe.sub.o e'tanS e.sup.2
where f is the frequency, e.sub.o is the permittivity of free space, e'tanS is the loss-factor of the thermoplastic and E is the electrical field strength. When the electrodes are placed in contact with the thermoplastic, e.sub.o is not a factor. The loss-factor is a characteristic of the thermoplastic (or the material added to the thermoplastic). The frequency, f, is chosen on the basis of the loss-factor over the temperature range of interest. The electrical field is limited by the dielectric breakdown voltage, i.e. the voltage at which the system will arc.
A process need be developed that minimizes waste in both the material that is used and the energy that is required to form the laminated billets. Such a process should not require the regrind and subsequent mix of all the materials of the sheets and the use of a regrind layer in the article. Additionally, such a process should not require the heating before lamination of the entire area of all of the sheets, including the area of the sheets that will not be used to form the laminated billets.